1. Field of the Invention
The present invention relates to a solid state image sensor wherein a photodetector array including photodetectors are arranged in two-dimension,.and a driving method therefor, or in particular, to a driving circuit of the sensor and a driving method therefor.
2. Description of the Prior Art
A solid state image sensor of charge sweep device (CSD) architecture comprises a two-dimensional photodetector array, vertical charge transfer elements each connected through transfer gates to a vertical line of the photodetectors, a pixel line selector for selecting one of transfer gates for a vertical line, and a horizontal charge transfer element (horizontal CCD) connected to the vertical charge transfer element through a storage gate and a storage control gate. A pixel line selector supplies a signal for designating a horizontal line to the transfer gates, and the signal charge in a photodetector is transferred by a transfer gate to the vertical charge transfer element. The vertical charge transfer element comprises gate electrodes in correspondence to horizontal lines, and a driving circuit supplying driving clocks to the gates to transfer the signal charge successively along the vertical charge transfer element. The storage gate stores temporarily the signal charge transferred from the vertical charge transfer element, and the storage control gate controls the signal charge transfer from the storage gate to the horizontal CCD. The solid state image sensor of charge sweep device (CSD) architecture is disclosed in M. Kimata, U.S. Pat. No. 4,581,539 and M. Kimata, U.S. Pat. No. 4,577,233. The structure and operation of the charge sweep device is also explained in M. Kimata, M. Denda, N. Yutani, S. Iwade and N. Tsubouchi, IEEE Journal of Solid State Circuits, Vol. SC-22, 1124-1129 (1987), and M. Kimata, M. Denda, N. Yutani, S. Iwade and N. Tsubouchi, Proceedings of SPIE, 930, 11-25 (1988).
It is known that the transfer gates and the gate electrodes are constructed as common electrodes. The driving of the vertical charge transfer elements is also possible with examples different from the above-mentioned one, or the pixel line selection may adopt interlace scan.
The driving circuit for the vertical charge transfer elements may comprise shift registers. However, the driving circuit may use four-phase driving clock signals applied according to a driving pattern to each four successive gates. If such a driving circuit comprises shift registers, when input states of the driving pattern to the shift registers are turned on and off repeatedly, and the driving pattern changes for an output cycle for each pixel. Then, an input pattern for the shift registers are needed as external driving clock signals, and this generates noises of a fixed pattern in an image.
In order to remove the noises of a fixed pattern, when four-phase clock signals for the vertical charge transfer elements are generated in the solid state image sensor, the driving circuit uses external four-phase clock signals. In principle, it is possible to drive the vertical charge transfer elements directly by using the external clock signals without inverting to the internal clock signals. However, the number of the gates in the vertical charge transfer elements is very large, and if all the gates are driven directly, a circuit of very large driving ability is needed, while vertical charge transfer has to be carried out at a fast speed in a horizontal period. Therefore, the direct driving is impossible actually.
In the above-mentioned driving circuit using four-phase clock signals, noises due to a fixed pattern can be decreased. However, each output of the driving circuit has to generate clock signals of at least a number of the pixels aligned along the vertical direction divided by four, and the potentials of all the gates are changed at each clock cycle. This operation corresponds to charging or discharging of a large capacity at a fast rate, and the electric power consumed by the vertical charge transfer elements becomes very large.
In principle, it is possible to decease the consumption power without affecting other characteristics by changing the driving method for the vertical charge transfer elements from the above-mentioned four phases to a larger phases. However, because a number of clock signals supplied from the external increases due to the multi-phase driving, a number of electrical conduction lines connected to the external also increases, and they act as thermal sources or heat conduction means. Therefore, it is difficult to apply the multi-phase driving especially to a solid state image sensor for infrared rays which has to be cooled.
On the other hand, another driving method is proposed where a start signal of the shift registers is supplied from the external, and driving clock signals are generated successively after an start signal is received. Though an output signal has noises due to the start signal, the start signal is supplied only once per horizontal period, and it may be supplied in a horizontal blanking period. Therefore, the noises due to the start signal are not a problem on an image.
In the above-mentioned driving method, a number of lines which can be charged or discharged at each clock cycle without increasing a number of necessary external clock signals is always one, and a consumption power can be decreased. However, the driving method has a problem that the transfer efficiency decreased because the signal charge disperses in the vertical charge transfer element.
In the vertical charge transfer of the prior art solid state image sensor of CSD architecture explained above, a low consumption power of the sensor and high efficiency of charge transfer cannot be realized at the same time.